Methamphetamine (MA) abuse and dependence are significant public-health concerns. Treatment admissions for MA use increased at an alarming rate from 1998 to 2007 in the US. Behavioral treatments reduce MA use. However, many patients enrolled in behavioral treatment programs are unable to achieve a significant period of abstinence suggesting other strategies like pharmacotherapy are urgently needed. G-Aminobutyric-acid (GABA) and opioid systems modulate dopamine. GABAA receptor modulators (e.g., oxazepam [OXP]) and opioid antagonists (e.g., naltrexone [NTX]) attenuate the abuse-related effects of amphetamines. The attenuation of the abuse-related effects of amphetamine by GABAA receptor modulators or opioid antagonists, while statistically significant, is modest in magnitude. Novel strategies are needed to enhance the efficacy of these drugs. Targeting GABA and opioid systems simultaneously is an innovative strategy in that combining NTX and OXP may produce greater attenuation of the abuse-related effects of MA. A rigorous within-subject experiment will be conducted in non-treatment-seeking, MA-abusing participants. NTX (0 and 50 mg/day) and OXP (0 and 40 mg/day), alone and in combination, will be tested with MA (0, 10, 20 and 30 mg). The four NTX-OXP maintenance conditions will be tested in random order. Similarly, within each NTX-OXP condition, the MA doses will be tested in random order. The reinforcing effects of intranasal MA doses will be determined after four days of maintenance on each of the NTX-OXP conditions using a sensitive progressive-ratio procedure. The ability to attenuate the reinforcing effects of drugs is a reliable predictor of an effective pharmacotherapy. We hypothesize that combining NTX and OXP will produce an additive or supra-additive reduction in the reinforcing effects of MA relative to the constituent drugs alone. This research will provide critical information regarding the initil efficacy a novel drug combination, NTX and OXP, for MA dependence. Innovations of the proposed research include: 1) testing a combination of marketed drugs that demonstrated some efficacy when tested as mono-therapies; 2) testing a GABAA receptor modulator that has minimal abuse potential and dependence liability, which will likely be more acceptable to clinicians; 3) the use of a sensitive drug self-administration procedure; 4) providing the impetus for the conduct of a Phase II clinical trial to further demonstrate the efficacy of NTX-OXP combinations for MA dependence; and 5) demonstrating the initial efficacy of commercially available drugs, as opposed to waiting for novel molecules to be available for testing in humans, thereby impacting clinical research and practice more quickly. In these ways, the proposed project will shift the current clinical research paradigm in pharmacotherapy development and have a significant impact on the treatment of MA dependence.